As is well known, an electric power steering apparatus is to assist a driver's operation of steering wheels in such a way that a torque sensor detects steering torque exerted on a steering wheel by a driver of a vehicle, a motor produces driving torque corresponding to the detected steering torque, and then the driving torque produced by the motor is exerted on a steering mechanism of the vehicle.
In general, an electric power steering apparatus is configured in such a way as to provide a phase delay or a phase advance to a torque signal from a torque sensor so that the gain is adjusted. For example, in a conventional control device for an electric power steering apparatus disclosed in Patent Document 1, a first-stage phase advance compensator formed of hardware applies phase-advance compensation to a torque signal from a torque sensor at a frequency around the crossover frequency of a steering system; then, a second-stage phase advance compensator formed of software applies phase-advance compensation to the torque signal in a low frequency band that is mainly utilized in practical steering; furthermore, a third-stage phase delay compensator applies phase-advance compensation to the torque signal so that the phase margin further increases. In the foregoing control device for an electric power steering apparatus, each of the second-stage phase advance compensator and the third-stage phase delay compensator is formed of software for a microcomputer.
In a conventional electric power steering apparatus configured in such a manner as described above, because the gain increases in a frequency band that is utilized in practical steering, the responsiveness of the steering system can be raised and the phase margin also increases, so that the stability of the steering system can also be enhanced.